Reactive intermediates carbanions, characteristics

Computational studies have compared substituent effects on the stability of ketenes, allenes, diazomethanes, diazirines, and cyclopropenes. Ketenes belong to the first generation of reactive intermediates along with carbocations, carbanions, radicals, and carbenes, and are intensively studied members of the cumulene family, with many useful synthetic applications. Ketenes were first recognized in 1905, when diphenylketene, a stable and isolable example, was obtained from the dehalogenation of the a-bromodiphenylacetyl bromide (Scheme 7.37). The most characteristic reaction of ketene is cycloaddition, as in the formation of p-laclams. 

Methods of obtaining block copolymers by radical processes have been developed rather lately about other processes, and especially ionic methods. This may be due to the nature of the radical, which is an intermediate with a very short lifetime, and a very high non-selective reactivity. These characteristics do not favor a well-controlled architecture as in the case of living carbanions appearing in anionic polymerization. However, the recent development of new... 

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